FMCW radar sensors are used in motor vehicles to detect the traffic surroundings, in particular for locating other vehicles. The location results may be used for various assistance functions, for example, for adaptive cruise control, an automatic collision warning or also the automatic triggering of an emergency braking operation in the event of an imminent collision risk.
The frequency of the intermediate frequency signal corresponds to the frequency difference between the signal sent at a given point in time and the signal received at the same point in time. Due to the frequency modulation of the transmission signal, this frequency difference depends on the transit time of the signal from the radar sensor to the object and back and thus on the distance of the object. However, due to the Doppler Effect, the frequency difference also includes a component, which is due to the relative velocity of the object. The measurement of the frequency difference on a single ramp therefore does not yet allow a determination of the distance and the relative velocity, but instead yields only a linear relationship between these variables. This relationship is represented as a straight line in a distance/velocity diagram (R-v diagram).
To obtain unambiguous values for the distance and the relative velocity, a conventional type of FMCW radar works with alternating rising and falling frequency ramps. In the R-v diagram, this then yields a different straight line for each ramp, the distance and the relative velocity of the object being defined by the point of intersection of these two straight lines.
However, when several objects are located simultaneously, the frequency spectrum of the intermediate frequency signal contains multiple peaks on each ramp, one for each object, and when the peaks on different ramps are compared, it is no longer possible to ascertain unambiguously which peak belongs to which object. For example, with simultaneous location of two objects, an R-v diagram having four intersecting straight lines is obtained. Only two of the four points of intersection yield the distances and relative velocities of the two objects, while the two other points of intersection represent so-called “phantom targets.”
To eliminate this ambiguity, at least one third frequency ramp, which has a different slope and a different set of straight lines in the R-v diagram, is used in most cases. The true objects may then be recognized by the fact that all three straight lines pass through the same point.
However, with an increase in the number of simultaneously located objects, there is a drastic increase in the probability that three straight lines will intersect randomly at almost the same point, whereby the effort also increases to resolve the ambiguities. Additional frequency ramps are often used to resolve ambiguities more easily.
The method described in the introductory paragraph represents an alternative approach to solving this problem. This method works with a series of identical, relatively short frequency ramps, so-called “rapid chirps,” which have a high frequency deviation in relation to their duration, and therefore are so steep that the distance-dependent component is dominant in the intermediate frequency signal, while the Doppler component represents only a minor correction. This correction is determined by the fact that the phase change of the intermediate frequency signal is monitored from one ramp to the next. This makes use of the circumstance that the phase of the intermediate frequency signal has a relatively sensitive response to the minor change in the object distance which results from the relative movement of the object during the short time interval from one frequency ramp to the next.
However, since the phase change is a periodic function of the relative velocity, the relative velocity may be determined unambiguously only when it is so small that the phase change amounts to less than half a period (i.e., less than π).
However, when using an FMCW radar in a motor vehicle, the relative velocities may be so high that this condition is violated. To nevertheless obtain unambiguous results, the duration and thus the repeat frequency of the chirps would have to be shortened further. However, this would not only require more computation power but would also entail a greater fuzziness in the distance measurement due to the shorter “observation period” accordingly and ultimately no measurement at all would be possible if the ramp duration is shorter than the time required for the radar signal to travel to the object and back.